In the first place, we would like to stress that it is not necessary that the number seven in the Prophetic hadith referred to in the question be understood literally. Rather, some commentators on the hadith state that eating in seven intestines is a rhetorical way of saying “eating too much or being too greedy.”
Dr. Muzammil H. Siddiqi, former President of the Islamic Society of North America and Director of the Islamic Society of Orange County, Garden Grove, California, states the following: “The author [of the flyer] quotes the Prophetic hadith in which the Prophet (peace and blessings be upon him) is reported to have said, “A believer eats in one intestine whereas a non-believer eats in seven intestines.” This is an authentic hadith and is quoted by several authorities. The scholars of Hadith explain this to mean that a true believer is a contented person. He takes little from the material world. The non-believer is greedy and is never satisfied with what he has. The scholars say that one should not take the number seven literally; it is used in Arabic to indicate large number or quantity. “Eating in seven intestines” is a metaphorical way of saying “eating too much or being too greedy.”
Some scholars say that the Prophet (peace and blessings be upon him) was not speaking generally but he was referring to a particular person who came to visit him. This person was offered a glass of milk to drink, and he drank. His host poured some more milk in his glass, and he drank that. Then some more was given, and he drank that, too. This continued until this person drank seven glasses of milk. Next day he came again to the Prophet and the Prophet spoke to him about the message of Islam. The visitor accepted Islam. The Prophet again offered him the milk, but this time he took only one serving and that was sufficient for him. The Prophet then remarked that when he was a non-believer he drank seven times more than what he drank after his accepting Islam. So, in this sense, some scholars say that this was not a general remark about every non-believer and believer. This remark was made about a particular person only. (See Tuhfatul Ahwazi Sharh At-Tirmidhi)”